1. Field of the Invention
The present invention relates generally to an actively controlled suspension system for an automotive vehicle for suppressing vehicular attitude change and absorbing road shock for achieving both of riding comfort and driving stability at satisfactorily high level. More specifically, the invention relates to a hydraulic circuit for the actively controlled suspension system, which hydraulic circuit is designed for saving power consumption required for generating fluid pressure for controlling suspension characteristics.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987 which has been assigned to the common owner to the present invention, discloses one of typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder is communicated with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as an proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit to be controlled the valve position. The pressure control valve is controlled the valve position by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and whereby controlling suspension characteristics.
On the other hand, European Pat. Nos. 0 283 004, 0 285 153 and 0 284 053 discloses technologies for controlling the suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body.
In one of the typical construction of the hydraulic circuit includes a pressure source unit which comprises a fluid pump drivingly associated with an automotive internal combustion engine so as to be driven by the engine output torque. The fluid pump is generally rated to produce rated pressure which is selected in view of the required line pressure in a supply line for supplying the pressurized fluid to the working chamber, at the minimum revolution speed of the engine so that the working fluid pressure to be supplied to the working chamber of the hydraulic cylinder can be satisfactorily high at any engine driving range. As will be appreciated, the output pressure of the fluid pump increases according increasing of the engine revolution speed. Therefore, at high engine revolution speed range, excessive pressure in excess of a predetermined maximum line pressure is relieved via a relief valve. Therefore, the engine output can be wasted to degrade engine driving performance as a power plant for the automotive vehicle and thus degrade fuel economy.
On the other hand, in the practical operation of the actively controlled suspension system, the fluid pressure in the working chamber in the hydraulic cylinder can be maintained at constant value for maintaining a desired vehicular height, at substantially low vehicle speed range or while the vehicle is not running. Despite this fact, the prior proposed hydraulic circuits for the actively controlled suspension systems supply the rated pressure of the fluid pump which should be higher than a minimum line pressure required for adjustment of the fluid pressure in the working chamber. In order to maintain the rated pressure to be output from the fluid pump, substantial engine output will be consumed even at the low vehicle speed rage, in which the line pressure is not required for no possibility of adjustment of the suspension characteristics.